DE4031948A1 - Aluminium:titanate prepn. - involves mixing aq. acidic titanium-contg. soln. with aq. alkaline aluminate soln., removing ppte., calcining, compressing and sintering - Google Patents

Abstract

Prepn. of aluminium titanate (I) comprises: (i) mixing an aq. acidic titanium-contg. soln. with an aq. alkaline aluminate soln. at pH 4-8 (5-7) to ppte. hydrated titanium oxide and hydrated aluminium oxide at 20-100 deg.C; (ii) removal and washing of the pptes; (iii) calcination of the pptes. at 400-1200 (700-900) deg.C to form a powdered mixt. of water-free Ti02 and Al203; (iv) partial deagglomeration of the oxide mixt.; (v) compression of the mixt. at 200N/mm2 to form a green moulding with a relative density of at least 40 pref. 50%; (vi) reaction sintering of the moulding above 1300 deg.C to produce aluminium titanate. USE/ADVANTAGE - (I) is used as an insulating material in IC engines. The raw materials are used to make (I) are inexpensive

Description

The present invention relates to a process for the preparation of aluminum titanate.

Aluminum titanate (Al ₂ TiO ₅ ) is ver used because of its high melting point, its low thermal expansion and the good thermal shock resistance for high temperature ceramics. Moreover, the sintered aluminum titanate is easy to machine, making it an important material for use as an insulating material in internal combustion engines, such as in the automotive field; see. z. B. "Properties and applications of aluminum titanate", P. Stingl (Hoechst Ceram Tec), in Proceedings Tech ceram 1987, Frankfurt am Main, p 11.01-11.06.

Aluminum titanate ceramic is polycrystalline and consists of grains ners that have a high degree of thermal anisotropy. Cooling of high temperatures causes stress ent long grain boundaries, resulting in the formation of microcracks and ge thermal expansion leads. The microcracking ver reduces the mechanical strength when the aluminum titanate grains in the sintered body is not a critical size of min reach at least 3-4 microns and a maximum grain size of about 10 μm is not exceeded. In this grain size range is a low thermal expansion with a minimum loss guaranteed to mechanical strength; see, for. B., "Cor Relation Between Grain Size and Thermal Expansion for Aluminum Titanate Materials ", F. J. Parker and R. W. Rice Communications of the American Ceramic Society, Vol. 72, No. 12, pp. 2364-2366 (1989).  

The starting powder is one of the most critical factors in the Production of structural ceramics based on aluminum titanate ba Sieren. It must be very pure and homogeneous and a small par have particle size.

Aluminum titanate is usually made from a stoichiometric Mixture of alumina and titanium oxide powder by conventional Ceramic manufacturing process such. B. co-grinding with followed mechanical or isostatic pressing and reaction sintered at temperatures <1300 ° C produced.

However, such manufacturing processes are subject to a number of disadvantages and are incorporated in their capacity limits. The two-phase powder may be at for example, homogeneity and uniformity on mikrosko There is a lack of a common level, which leads to a lack of compaction sintered body leads.

By chemical processes involving co-hydrolysis of aluminum and titanium alkoxides or hydrolysis of titanium alkoxides in the presence of fine aluminum oxide seed particles, very fine powders can be prepared which are homogeneous at the microscopic level. If such powders are sintered at temperatures of <1300 ° C., the result is aluminum titanate bodies with a high sintering density; see. z. "Preparation and Sintering of Narrow-Sized Al ₂ O ₃ -TiO ₂ Composite Powders", H. Okamura, EA Barringer and H. Kent Bowen. J. of Materials Science 24 (1989), p. 1867-1880.

This procedure requires the use and handling of potentially dangerous organic substances. simultaneously The costs of the starting materials are so high that one on This made aluminum titanate no industrial Can gain meaning.

Furthermore, it is known to produce reactive titanium and aluminum oxides, which have a homogeneous mixture in the microscopic range, by direct evaporation of an aqueous solution of aluminum sulfate and titanyl sulfate with subsequent decomposition of the dried sulfates in the reactive oxides (anatase and γ-alumina) Calcining at a temperature of about 800 ° C. High density aluminum titanate is obtained by sintering at temperatures above 1300 ° C. The density of the sintered bodies produced is higher than that which can be achieved with the aforementioned methods. However, in this case the addition of auxiliaries, such as. As SiO ₂ or MgO, necessary to target a Aluminiumtitanat with small grain size and correspondingly low thermal expansion values to it; see. z. B. "Fabrication of Al ₂ TiO ₅ Ceramics: 1. Synthesis of Al ₂ TiO ₅ from Mixed Sulfate of Al and Ti", E. Kato, K. Daimon, J. Takahashi, R. Kato and K. Hamano. Report of the Research Laboratory of Engineering Materials, Tokyo Institute of Technology, No. 9, pp. 75-86 (1984).

The object of the present invention is to provide a method for To develop production of aluminum titanate in which the ge called disadvantages of the known methods are avoided. The new process should allow it, from inexpensive starting materials in an intermediate stage a fine crystalline powder mixture very homogeneously distributed, reactive aluminum and titanium oxides produce. This should make further processing into a pure, High density aluminum titanate with low thermal emission elongation and high strength properties on simple and cost-effective way possible.

This object is achieved by a method with the features of the main claim. Advantageous Ausgestal tions of the method will become apparent from the dependent claims.

According to the method of the invention, an aqueous acidic titanium-containing solution and an aqueous alkaline aluminate solution are mixed together. The pH of the mixed solution is maintained in a range between 4 and 8. At this pH range, alumina and titania together precipitate out of solution in hydrated form. The precipitated solids are separated from the solution and then washed. The resulting precipitated product has a very homogeneous distribution of titanium and aluminum. In the subsequent process step, the precipitate is calcined at a temperature between 400 and 1200 ° C. The hydrated oxides are converted into the water-free, crystalline oxides TiO ₂ or Al ₂ O ₃ . The product is a homogeneous, powdery mixture of these oxides with fine primary crystals, which are agglomerated into larger particles. In order to ensure a sufficiently high pre-compaction during the subsequent pressing of the oxide mixture, a Deagglo is meration of the particles necessary. According to the proposed method, only a partial deagglomeration is sufficient. After this grinding process, the oxide powder mixture is pressed into a green shaped body having a relative density of at least 40%. Subsequently, a reaction sintering of green shaped body takes place at a temperature above 1300 ° C. The oxides react with each other to form Aluminiumti tanat. It is produced a high-density aluminum titanate, which has a high purity and is virtually free of unrea cited titanium or alumina. The grain size in the product obtained is between 4 and 10 microns, so that the product has not only a low thermal expansion and the desired high mechanical strength.

The starting solutions for the process according to the invention can be prepared using inexpensive commercial chemicals. As titanium-containing solutions are z. B. solu- tions of TiCl _4, Ti (SO _{4) 2,} or TiOSO ₄ in water. The use of chloride solutions is advantageous here, since chlorides are easier to wash out of the precipitate than are removed at a lower calcination temperature than is the case with corresponding sulfates. The aluminate solution can be very simple z. B. be prepared by dissolving Al (OH) ₃ in aqueous solutions of NaOH, LiOH or KOH, with the corresponding aluminates NaAlO ₂ , LiAlO ₂ or KAlO ₂ form. To prepare the solutions, deionized water is preferably used to prevent the introduction of impurities.

In order to obtain a homogeneous as possible precipitate product should Starting solutions are mixed intensively with each other. This  can either be done directly or z. B. by common A pass the two solutions into a high-stirred receiver Water. Again, it is appropriate to deionized water use to avoid contamination.

Compliance with the specified pH range of 4 to 8 in the Mixed solution is essential to a simultaneous and possible complete precipitation of the hydrated oxides in homogeneous to ensure distributed form. If this area under is stepped, the titanium hydroxide precipitates only incomplete. If this range is exceeded, this applies to the Aluminum hydroxide. The best results are obtained in a pH Range from 5 to 7 received. The setting of the required pH can be z. B. by addition of acids or alkalis in Ab depending on the current pH of the solution.

The composition of the solutions and / or the ratio of the amounts of solution used is selected so that in the precipitate a nearly stoichiometric ratio between Al and Ti is present, based on the subsequent conversion to Al ₂ TiO ₅ .

The precipitation reaction can basically in a wide Tempe temperature range. However, with tempera tures below 20 ° C a costly cooling of the system necessary. At temperatures above 100 ° C, the boiling point of the Solutions are achieved. At such high temperatures is the System unnecessarily supplied energy without optimizing the Ver to achieve.

The precipitated solid can very easily by filtration, Sedimentation or centrifugation separated from the solution become. The precipitate is then washed to remove chloride. or sulfate components of the adhering solution. For this purpose, again deionized water should be used to To avoid contamination.

During the subsequent calcination, the hydrated oxides are converted into the corresponding substantially anhydrous, crystalline oxides. These oxides should be as reactive as possible in order to obtain a rapid reaction to Al ₂ TiO ₅ during the later reaction sintering. The reactive oxide forms are anatase and γ-Al ₂ O ₃ . Basically, the calcination can be carried out in a temperature range between 400 and 1200 ° C. Below 400 ° C., complete conversion of the hydrazated oxides into the largely anhydrous oxides is not guaranteed. Above 1200 ° C., a partial reaction of the oxides to Al ₂ TiO ₅ and partial sintering already starts. The preparation of a high-density single-phase Aluminiumti tanats is then no longer possible. The preferred temperature range for calcination is between 700 and 900 ° C. It should be noted that above 800 ° C already intensified the conversion of anatase in rutile and of γ-Al ₂ O ₃ in α-Al ₂ O ₃ begins. These less reactive oxide forms cause a significant delay in the formation of Al ₂ TiO ₅ and can thus lead to undesirable levels of unreacted oxides or to an inhomogeneous product.

Adhering sulfate residues can only reach temperatures of 800 ° C going to be completely removed. When using Sul fat solutions should therefore have a calcination temperature between 800 and 900 ° C are set. When using chloride solutions if a calcination temperature between 700 and 800 ° C is expedient, to avoid the mentioned disadvantages.

After calcination, the oxides are in the form of very fine pri märkristalle that are agglomerated into larger particles. In the subsequent precompression step, the highest possible Density of the green shaped body are generated at the time of closing reaction sintering a high-density aluminum to obtain titanate product. Therefore, a deagglomeration is the Particles required before the precompacting step. It has but surprisingly shown that in the application of the process according to the invention only a partial Deagglo The particle size is sufficient to ensure a sufficiently high density to reach the green shaped body. As aggregates for the par Partial deagglomeration are suitable for. B. ball, roller, Vibrating or air jet mills. After the grinding process should the average particle diameter preferably in the range below 10 microns lie.  

In the subsequent compression stage, the partially zer kleinerte oxide powder mixture is pressed ver to a green shaped body. The relative density of the green body must be min least 40%. Preferably, it should be above 50%. Due to the high density, a sufficient particle contact is ensured, whereby a fast, complete reaction of the TiO ₂ and Al ₂ O ₃ particles to Al ₂ TiO _{5 is possible} during the sintering sintering process.

In the final reaction sintering, the conversion of the oxides into Al ₂ TiO _{5 takes place} with densification of the structure at temperatures above 1300 ° C. If the green shaped body is heated too slowly to the reaction temperature, the less reactive oxides (rutile and α-Al ₂ O ₃ ) can be formed as intermediates with the disadvantages already mentioned. The heating rate should therefore be at least 10 ° C / min, preferably at least 15 ° C / min in this process stage.

In this way, it succeeds, a high-quality aluminum titanate with the desired properties. The proposed gene procedure is inexpensive and easy in the through guide. There are neither consuming evaporation steps neces still, the process requires the use of additional Excipients.

The method according to the invention will be described below with reference to a Embodiment explained in more detail.

embodiment

As starting solutions, an aqueous TiCl ₄ acid solution and an aqueous alkaline sodium aluminate solution were used. The solutions were prepared from the commercial chemicals TiCl _4, Al (OH) ₃ and NaOH. To prepare the TiCl ₄ solution containing 129 g / l TiCl ₄ , liquid TiCl _{4 was} slowly introduced into deionized water. The result was a white cloudy solution, which became clear after standing for 24 hours at 4 ° C. The sodium aluminate solution containing 130 g / l Na ₂ O and 68.4 g / l Al ₂ O ₃ was prepared by dissolving Al (OH) ₃ in a dilute NaOH solution.

In a 2 liter beaker with 300 ml of deionized water was added generated by means of a magnetic stirrer a vortex funnel. ever because 500 ml of the two prepared solutions became the same early and slowly introduced into the water, in the immediate area of the vortex funnel. The pH of the ent standing mixed solution was monitored and by addition of 3N HCl (100 ml total) in a range between 5 and 7 held. Under these conditions, titanium oxide and aluminum fall miniumoxid in hydrated form together from the solution. Upon completion of the precipitation reaction, the solids were removed filtered and washed with 4 liters of deionized water. That so obtained precipitate was dried at 110 ° C and at then analyzed. Table 1 shows the analysis values of Precipitated product.

Analysis of the precipitate Mass % TiO₂ 26.7 Al₂O₃ 32.3 SiO₂ 0,190 Fe₂O₃ <0.005 Na₂O 0,035 Cl⁻ 3.32 Loss on ignition (1100 ° C / 1 h) 40.6 Specific surface area (m² / g) 19.5

The specific surface of the dried precipitate was measured by nitrogen adsorption (BET one-point method; DIN 66 132).

The dried precipitate was also examined with the scanning electron microscope ( FIG. 1), the combined REM / EDX X-ray analysis ( FIG. 2) and the X-ray diffraction ( FIG. 3). The particle size distribution is shown in FIG. 4.

The primary crystal size is between 0.05-0.10 μm, the crystals being present in agglomerated structures with an average particle size of 37 μm. The mixed powder is nevertheless X-ray amorphous ( FIG. 3).

The dried precipitate was then placed in an electric Kiln (Allio) at a temperature of 800 ° C for 1 hour normal atmosphere calcined. Table 2 shows the analysis of calcined precipitate.

Analysis of the precipitate after one hour calcination at 800 ° C. Mass % TiO₂ 42.2 Al₂O₃ 51.6 SiO₂ 0.26 Fe₂O₃ <0.005 Na₂O 0,041 Cl⁻ 0.24 Loss on ignition (1100 ° C / 1 h) 5.0 Specific surface area (m² / g) 65.9

Fig. 5 shows a scanning electron microscope photograph of kalzi-based material, Fig. 6 shows the EDX analysis and Fig. 7, the Rönt gen diffraction analysis.

Although the direct precipitate is X-ray amorphous (as shown in Figure 3), heating at 800 ° C for one hour results in a crystalline powder in which anatase (with some rutile) is the predominant titanium oxide phase. Figure 7 shows the parallel evolution of the apparently less crystalline and somewhat porous γ-Al ₂ O ₃ . Scanning electron micrographs indicate an increase in primary crystal size to 0.1-0.2 μm as a result of one hour calcination at 800 ° C.

EDX analysis confirmed the homogeneity of the calcined precipitate with respect to the distribution of titanium and alumina within the agglomerated particles ( Figure 6).

After calcination, the resulting oxide powder mixture was partially deagglomerated in a vibration mill using alumina milling media for a milling time of 30 minutes. Fig. 8 shows the grain size distribution of the ground powder; the average grain size was reduced to 5.1 microns ver.

The milled powder mixture was then uniaxially pressed at 200 N / mm ² into moldings of about 20 mm diameter, about 5 mm thick and about 5.0 g in weight. The thus precompacted compacts were air dried at 110 ° C for 24 hours. From the dry weight and the molded body dimensions, a press density of 2.13 g / cm ^{3 was} determined. This gives a relative density of about 58%, a high value considering the partially agglomerated state of the starting powder.

The green moldings were air-dried for 4 hours at 1400 ° C in an electrically heated Netzsch oven with programmable Temperature control reaction sintered. The heating speed To 1400 ° C was 17 ° C per minute. The sintered Moldings were cooled in situ in the oven.  

After sintering, the apparent sintered density was measured with the aid of Archimedes method and calculated the open porosity net. Table 3 gives the information for the green and the Gesin shaped body.

Properties of green and sintered moldings Press density (g / cm³) 2.13 After sintering: @ Water adsorption (%) 2.00 Sintered density (g / cm³) 3.39 Open porosity (%) 6.80 Apparent density (g / cm³) 3.64 relativ density (%) 92.4 Linear relative shrinkage (%) 17

The linear relative shrinkage caused by the sintering was measured at 17%, at 92.4% of the theoretical density. The shrinkage is due to the combined effects of sintering and the phase changes experienced by the two-phase powder (Al ₂ TiO ₅ has a slightly lower density than a corresponding mixture of α-alumina and Ru til).

The microstructure of the sintered moldings was examined by means of a scanning electron microscope ( Figure 9), topo chemical analyzes were made by EDX ( Figure 10), and the crystalline phases were determined by X-ray diffraction ( Figure 11).

The X-ray diffraction analysis shows that the sintered Pro is an aluminum titanate consisting of one phase without Indication of the presence of unreacted titanium or alumi niumoxid.  

The scanning electron micrograph ( Figure 9) shows the presence of crystallographic domain formation, with the cracks likely to be due to anisotropic thermal shrinkage.

The crystal size in the sintered body is between 4 and 10 μm. This will set the conditions for low thermal Expansion and good mechanical strength of the produced Alumi met Titanit.  

List of pictures

Fig. 1 Scanning electron micrograph (SEM) of ge dried precipitate

Fig. 2 EDX analysis (energy dispersive X-ray analysis) of the dried precipitate

Fig. 3 X-ray diffraction analysis of the dried precipitated product

Fig. 4 grain size distribution of the dried precipitate

Fig. 5 Scanning electron micrograph of the oxide powder mixture after one hour calcination of Fällpro product at 800 ° C.

Fig. 6 EDX analysis of the oxide powder mixture after calcination of the precipitate at 800 ° C for one hour

Fig. 7 X-ray diffraction analysis of the oxide powder mixture after one hour calcination of the precipitate at 800 ° C.

Fig. 8 grain size distribution of the oxide powder mixture after an hourly calcination of the precipitate at 800 ° C and subsequent partial deagglomeration

Fig. 9 Scanning electron micrograph of the sintered aluminum aluminum titanate

Fig. 10 EDX analysis of the sintered aluminum titanate

Fig. 11 X-ray diffraction analysis of the sintered Alumi niumtitanats.

Claims (13)

1. A process for producing aluminum titanate, characterized in that the process comprises the following process steps:

• a) mixing an aqueous acidic titanium-containing solution with an aqueous alkaline aluminate solution with adjustment of a pH of the mixed solution in the range between 4 and 8, wherein jointly hydrated Ti tanoxid and hydrated alumina are precipitated as solids;
• b) separating and washing the precipitated solids;
• c) calcining the precipitate at a temperature between 400 and 1200 ° C to a powdery mixture of the substantially anhydrous oxides TiO ₂ and Al ₂ O ₃ ;
• d) partial deagglomeration of the oxide mixture;
• e) pre-compacting the oxide powder mixture by pressing into a green shaped body having a relative density of at least 40%;
• f) reaction sintering of the green shaped body at a temperature above 1300 ° C, wherein the individual oxides are converted into aluminum titanate.

2. The method according to claim 1, characterized in that an aqueous acidic solution of TiCl ₄ , Ti (SO ₄ ) ₂ or TiOSO _{4 is} used as the titanium-containing solution. 3. The method according to any one of the preceding claims, characterized in that the aluminate solution used is an aqueous alkaline solution of NaAlO ₂ , LiAlO ₂ or KAlO ₂ . 4. The method according to any one of the preceding claims, characterized characterized in that the titanium-containing solution and the Alumi natlösung be intensively mixed by simultaneous Initiate the two solutions in a template with strong stirred water, preferably deionized water, wherein the pH of the resulting mixed solution by controlling Addition of an acid or alkali to a value between 4 and 8 is regulated. 5. The method according to any one of the preceding claims, characterized characterized in that the pH of the mixed solution to a Value is regulated between 5 and 7. 6. The method according to any one of the preceding claims, characterized characterized in that the process step a) at a Tem temperature between 20 and 100 ° C is performed. 7. The method according to any one of the preceding claims, characterized characterized in that the precipitated solids by sediments tation, filtration or centrifugation of the mixed solution be separated. 8. The method according to any one of the preceding claims, characterized characterized in that the separated solids with de be washed with ionized water. 9. The method according to any one of the preceding claims, characterized characterized in that the calcination of the precipitate according to Process step c) at a temperature between 700 and 900 ° C is performed.   10. The method according to any one of the preceding claims, characterized characterized in that the partial deagglomeration according to Process step d) in a ball mill, a roller mill, a vibrating mill or an air jet mill through to be led. 11. The method according to any one of the preceding claims, characterized characterized in that the oxide powder mixture in the process step e) to a relative density of at least 50% is pre-compressed. 12. The method according to any one of the preceding claims, characterized in that the pre-compression of the oxide powder mixture is carried out at a pressure of 200 N / mm ² . 13. The method according to any one of the preceding claims, characterized characterized in that the green molded body in the process step f) with a heating rate of at least 10 ° C / min., Preferably at least 15 ° C / min, to a Temperature is heated above 1300 ° C.